This invention relates to hydraulic control systems and more particularly relates to a valve system for reducing back pressure and preventing cavitation in a hydraulic actuator or motor caused by the influence of external forces on the actuator or motor.
Many hydraulic systems are used for the actuation of equipment involving both resisting and assisting loads during each half cycle of actuation. The actuation is also at a regulated speed and is limited at one end by physical stops. The specific actuation problem for which the valve system of the present invention was developed was the deployment of the leading edge devices on the wing of an aircraft, however it is equally applicable to hydraulic systems employed in earth moving vehicles and the like using double acting hydraulic jacks for adjusting various machine components and implements carried on such vehicles, for example the bucket of a front loader or the body of a dump truck.
In the environments mentioned above during the initial portion of the working stroke, the hydraulic actuator or motor works against the weight of the load which it is moving. However, at some point in the cycle the center of gravity of the load passes over the pivot axis thereof and for the remainder of the stroke the load is assisting, that is, the load tends to move the actuator in the same direction as the driving fluid supplied to the actuator. In order to maintain a constant speed of actuation, it has in the past been necessary to place a flow limiter in the outlet flow path from the hydraulic actuator cylinder to maintain a constant outflow of fluid from the cylinder. In most circumstances, however, it is undesirable to substantially restrict the flow of fluid to or from the actuator since such restriction causes the undesirable generation of heat and the build up of back pressure in the system which consumes energy that could be more beneficially used to perform other work functions desired of the system. However, if the load is allowed to move the actuator more rapidly than the design speed of the system, the inlet fluid pressure to the actuator can be reduced to a negative amount due to the load causing the actuator to move faster than the system can supply fluid to it. The negative pressure on the inlet side of the actuator may cause cavitation along the surface of the actuator piston which can cause severe damage to the components of the system. Other problems are presented by the use of a flow limiter in the outflow line to substantially limit the flow of fluid in the outlet side of the actuator during the movement of an assisting load. The back pressure on the outflow side of the actuator with the assisting load will be the sum of the pressure required to maintain the desired rate of motion plus full system pressure due to the fluid inletting to the inlet side of the actuator. Under many load levels this sum of pressures would easily exceed the maximum system pressure thereby requiring special hydraulic hardware. The excess back pressure can be avoided by applying return pressure to the inflow side of the actuator during assisting loads. This would, however, involve additional load sensing equipment, valving and other apparatus and would increase the complexity of the hydraulic system. Also, when the hydraulic system is being used to control the aerodynamic surfaces of an aircraft the addition of apparatus adds to the weight of the hydraulic system and therefore the total weight of the aircraft. Also the application of return pressure to the inlet side of the actuator would not necessarily solve the cavitation problems, since at return pressure the inflow could still cavitate causing actuator or motor difficulties.
Finally, with the conventional approach outlined above at the end of the actuation, the actuator would be positioned in the physical stops and would be loaded with full system pressure on the inlet side of the actuator cylinder placing a strain on the system and requiring the physical stops to be constructed so as to be sufficient to withstand the full system pressure, plus the weight of the load if the load is assisting, rather than some lower pressure which would allow a lighter construction of the physical stops.
It is therefore an object of the present invention to provide a valving system for use with a hydraulic actuator or motor that would minimize back pressure generated by a metered outflow regulating device under load conditions that vary from maximum resisting to maximum assisting during one-half cycle of operation of the actuator.
It is a further object of this invention to provide such a valving system that will prevent cavitation of the actuator or motor during extension of the actuator at a maximum rate with an assisting load.
It is another object of this invention to provide such a valving system that will provide a controlled predetermined level of load on the physical stops when the actuator reaches the end of its travel.
A further object of this invention is to provide such a valving system that is of simple construction and relatively inexpensive to construct and install.